Conventionally, when cutting hard and brittle workpieces such as silicon ingots, wire saws are used. As a method for cutting a workpiece with a wire saw, e.g., a method is disclosed wherein abrasive grain slurry is poured from nozzles arranged on both sides of the workpiece in lateral direction, onto a saw wire that is wound around a pair of wire guide rollers and moves left and right, while the workpiece is pushed against the saw wire and thus, the workpiece is cut [see, e.g., JP Patent Application Publication (Kokai) No. 7-1442 (hereinafter, JP '442”)]. Furthermore, a method for cutting a workpiece with a saw wire is disclosed wherein in the process of cutting the workpiece with the saw wire, the workpiece is immersed into abrasive grain slurry in a tank so that the whole area where the workpiece meshes with the saw wire is immersed [see, e.g., JP Patent Application Publication (Kokai) No. 2008-160124 (hereinafter, “JP '124”)]. Moreover, conventionally, a plurality of saw wires aligned in parallel are used to cut the In recent years, silicon ingots have become larger in diameter, so that precise cutting techniques for large-diameter silicon ingots are required. In the cutting method of JP '442, during the cutting run of the saw wire from the entry side to the exit side of the workpiece, i.e. in the meshing area of the saw wire, the abrasive grain slurry adhering to the saw wire and being supplied from nozzles is subject to attrition and separation and thus decreases. Therefore, in the case where a silicon ingot with a large diameter of 450 mm is cut, abrasive grain slurry sufficient for the cutting run of the saw wire from the entry side to the exit side of the workpiece cannot be provided only by supplying abrasive grain slurry to the saw wire from nozzles on both sides of the silicon ingot in lateral direction. Thus, decline, fluctuations, etc. in the cutting performance develop, and in the wafers formed by the cutting, undesired wafer properties such as undulation and bending occur, so that problems such as breakage of the saw wire emerge. As shown, e.g., in FIG. 8, in conventional cutting methods, the cutting margin (incision depth) of the saw wire on the exit side is much smaller than the cutting margin of the saw wire on the entry side. Incidentally, FIG. 8 is a drawing showing the enlarged cut surface of the left hand side part of the workpiece in the direction of the cutting run of the saw wire.
Furthermore, in the cutting method of JP '124, a surplus of abrasive grain slurry is supplied to the cutting part of the workpiece, the saw wire vibrates in the cutting groove, and the sliced part vibrates, so that the problem of wafers cracking occurs because neighboring wafers touch each other.
Thus, with conventional methods for cutting a workpiece with a wire saw it was impossible to cut silicon ingots with a large diameter of e.g. 450 mm without any problems.
Furthermore, since in the cutting method of JP '124 the whole area where the saw wire and the silicon ingot come into contact is immersed into an abrasive grain slurry liquid, particularly in the case of large diameter wafers, a surplus of abrasive grain slurry is supplied. Therefore, because the saw wire causes the slice parts to vibrate, or additionally because the abrasive grain slurry liquid inside the abrasive grain slurry bus is vibrated violently by the vibration due to the to-and-fro movement of the saw wire, the sliced parts vibrate under the influence of the vibration of the abrasive grain slurry liquid. Therefore, the quality of the cut surface regarding flatness, etc., decreases, and in some cases, problems such as the generation of cracks occur because neighboring sliced parts touch each other.